Nanoscale Surface
Characterization

Atomic ForceMicroscopes

www.witec.deWITecAtomic Force Microscopes

Nanoscale Surface Characterization

The WITec Atomic Force Microscope (AFM) module integrated with a research-grade optical
microscope provides superior optical access, easy cantilever tip alignment and highresolution sample survey. The WITec AFM objective provides a direct view of both sample
and cantilever for straightforward and precise AFM tip positioning. All WITec Atomic ForceMicroscopes are designed for combination with other imaging techniques such as Raman
spectroscopy, Scanning Near-field Optical Microscopy (SNOM), luminescence microscopy,
polarization analysis and light-field/dark-field illumination. The user can change between
imaging methods by simply rotating the (optionally motorized) objective turret.

Benefits• Surface characterization on the nanometer scale• Non-destructive imaging• Optical and Atomic Force Microscope combination• Convenient sample access from any direction• Minimal, if any, sample preparation• Ease of use in air and liquids• Combinable with confocal Raman imaging and Scanning
Near-field Optical Microscopy (SNOM)• TrueScan™ controlled piezo-driven scanning stages
with capacitive feedback loops:- 30 x 30 x 10 µm3- 100 x 100 x 20 µm3- 200 x 200 x 20 µm302The AFM Microscope

Photodiode
Laser

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AFM Sensor

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05

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06

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02
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Objectives
Piezo-driven scanning stage
Koehler white-light illumination
Color video camera
Z-stage for focusing
Active vibration isolation table

SampleSample Scanning

Beam Path

Working Principle

Atomic Force Microscopy (AFM) traces the surface contourof living and non-living samples by analyzing theinteraction forces between the sample’s surface and asharp cantilever tip. The sample is scanned under the tip
using a piezo-driven scanning stage and its topography
is displayed as an image with up to atomic resolution.

The topography of a sample can be acquired while
simultaneously extracting other properties suchas the sample’s adhesion, stiffness, viscosity,electrostatic potential and more.

WITec AFM | 03

TechnologyCantilever

Resolution

TrueScan™

Computer InterfaceAB• Inertial drive cantilever mount for AFM sensor positioning• All commercially available AFM cantilever tips can be used

The piezo-driven scanning stage moves the sample forward
and reverse during the measurement. The scanning stage’s
inertia leads to lag errors. This effect increases with scanningspeed. While the incorporated capacitive feedback systemdetermines the actual position of the stage, TrueScan™ canresolve the discrepancy between its actual and intended
position, allowing high-resolution lag-free AFM imaging athigh scanning speeds.

• Lateral resolution: Tip radius-dependent, down to 1 nm
• Depth resolution: Down to atomic scale on silicon,
mica, HOPG etc.

• alphaControl: Digital controller for WITec microscope systems• WITec Control software for instrument and measurement
control• Data evaluation and processing software included

Scan of the straight edge of a sample at speeds between 0.2 s and 1 s per scanning step. (A) Without TrueScan™ an increase inscanning speed leads to positioning errors. (B) TrueScan™ corrects the lag errors.

Simultaneous Cantilever and Sample Viewing for Easy Determinationof the Measurement Position
AB

219 nmCThe WITec AFM objective allows simultaneous sample and cantilever survey
from above with high resolution. It facilitates accurate and convenient AFM
tip alignment and positioning on even very small sample structures.
(A) Simultaneous cantilever and sample viewing.
(B) Overlay of the optical image with the AFM image(AFM image: AC mode, scan range: 3 x 3 μm2).
(C) 3D representation of the AFM image from B.

040 nmAFM Modes with DPFM
• Contact Mode
The cantilever tip is kept in contact with the sample surface at a constantbending (force) while it is scanned, revealing the topography of the sample.
The bending of the cantilever is monitored with a quadrant photodiodebeam-deflection system. In this imaging mode large lateral forces may
disturb loosely-bound particles.
• AC ModeIn this approach, also called intermittent contact mode, the cantilever
oscillates at its resonance frequency and is periodically in contact with thesample. Thus the technique is particularly well suited for delicate samples.
When the tip comes close to the surface, sample-tip interaction causes
forces to act on the cantilever which alter the phase of its oscillation.
This phase shift can be recorded and depicted as a phase image.
• Lift Mode™
This mode can be applied in combination with contact mode or AC mode.
First, the sample is scanned in an imaging mode to trace the surface.
Then Lift Mode™ is used to scan the sample again with a certain z-offset
following the previously-recorded topography. Lift Mode™ is used incombination with EFM, MFM or Kelvin Probe Microscopy to reveal sample
properties other than topography.
• Kelvin Probe Microscopy
In this imaging mode the conductive AFM tip acts as a vibrating
capacitor and measures in Lift ModeTM the differences in surface
potential between the tip and the sample.

• Digital Pulsed Force Mode™ (DPFM)
DPFM allows for the simultaneous acquisition of topography, adhesion,
stiffness and other physical properties of the sample. For details see thenext section.
• Magnetic Force Microscopy (MFM)
This imaging mode uses a magnetic cantilever scanned in Lift ModeTM
over the sample. During the z-offset scan, magnetic properties of thesample are revealed due to the magnetic interaction between the tip
and the sample.
• Electrostatic Force Microscopy (EFM)
In this imaging mode electrical properties of a sample surface areimaged. A DC voltage is applied between the tip and sample surface
while scanning in Lift ModeTM. When the tip is scanned at a z-offset overthe sample, electrostatic charges lead to deflections of the cantileverresulting in an electrical properties map of the sample.
• Nanomanipulation / Lithography
The lithography package “Da Vinci” enables the patterning of material
surfaces on the nanometer-scale using tip-sample interaction forces.
• Lateral Force Microscopy (LFM)
This imaging mode uses the twisting of the cantilever while scanningin contact mode to reveal surface friction characteristics.

Digital Pulsed Force Mode (DPFM)
Pulsed Force Mode (PFM) is a non-resonant, intermittent
contact mode for Atomic Force Microscopy that allows thecharacterization of material properties such as adhesion,
stiffness and viscosity along with the sample topography.
Additionally, lateral forces are virtually eliminated. Therefore
high-resolution mapping of delicate samples in air and liquids
is easily achievable while maintaining a scanning speed
comparable to contact mode AFM. In contrast to most otherintermittent contact techniques, the perpendicular forceson the sample (introduced by the AFM tip) are controlled by
the feedback loop. The PFM electronics induce a sinusoidal
modulation of the z-piezo of the AFM with an amplitude of
10-500 nm at a user-selectable frequency of between 100 Hz
and 2 kHz: far below the resonant frequency of the cantilever.
A complete force-distance cycle is carried out at this rate,
resulting in the force signal as shown in the figure below.

Imaging of Surface Properties with DPFM
yBaseline

Free Cantilever
Oscillation

y

Force

Fmax
Snap

Force SignalyAdhesionx
Stiffness
Adhesionx
Stiffness

Contact Time
Viscosity

ContactViscosityTimeContact TimexStiffness
Viscosity

Adhesion
PeakPulsed Force ModeTimeTopography

Digital Pulsed Force Mode (DPFM)

A selection of simultaneously-obtained
images of a Ethyl-Hexyl-Acrylate/Polystyrene blend (EHA-PS) spin-coated ontoa glass substrate. The corresponding
curves show the analyzed Pulsed ForceTopography
Mode properties. DarkTopography
areas correspondto low values. Scan range: 10 x 10 µm2.
WITec AFM | 05

Applications

Investigation of Electrostatic Characteristics
AFM images of a gold structure
measured in AC Lift Mode™ andwith Electric Force Microscopy(EFM) revealing (A) topographyand (B) conductive/nonconductive
areas of the sample.

A Topography

80 nm

B phase0 nm

High-resolution AFM Measurement
Image of mono-atomic steps of Highly Ordered Pyrolytic Graphite (HOPG).
Sample size: 500 x 300 nm2, total height in Z: 0.7 nm, height of single steps
between layers: 0.3 nm.

0.7 nm0.3 nm0 nm

0.3 nm06

Large-area Measurements in Liquids
A Topography

B(A) 250 x 100 µm2 large-area
topography scan of a cell culture
in liquid. Maximum measured
height: 2.5 µm.
(B) Water immersion objective for
AFM measurements in liquids.

Investigation of Magnetic Forces
Topography

Magnetic Force MICROSCOPY

Magnetic Force Microscopy (MFM)
measurement of a hard drive. The
measurements were performed
using AC mode with magnetic
tips. The topography is flat anduniform (see small image). The
MFM image of the same sample
area shows a clear magnetic
contrast between magnetic hard
drive domains (see large image).

4 µm

Temperature and Time Series
Topography 25° C

Topography 90° C

Topography 120° C

Topography 130° C

40 nm

0 nm

Adhesion 25° C

Adhesion 90° C

Adhesion 120° C

Adhesion 130° C

2.5 nN

0 nN

DPFM AFM images of heated paraffin at different temperatures.
Top row: Topography changes with rising temperature. At 130° C
the topography is flattened due to melting processes.
Lower row: The adhesion increases while the temperature rises.

WITec AFM | 07

Nanomanipulation
A human chromosome was first cut using the WITec DaVinci
nanolithography package and then imaged with AFM.
The zoom-in highlights the section.

< Cut126 nm

0 nm

Applications of Different AFM Modes for Comprehensive
Sample CharacterizationATopography

200 nm0 nm

A geological sample including
fossilized bacteria was analyzed.

Badhesion

The topography (A) does not allow
the identification of the fossilizedorganisms. However, measuring
the same area in Digital Pulsed
Force Mode reveals the presenceof the fossils by the differencein adhesion (B).
1 µm

08

60 nN

0 nN

Correlative AFM and Raman Study of a Polymer Blend
ATOPOGRAPHY

100 nmBPHASE

Digital Pulsed Force Mode (DPFM)
imaging demonstrates its mostimmediate advantage: topography
and other physical information canbe acquired simultaneously.

0 nmCAdhesion

30 nN

DDigital Pulsed
Force Curves

10 nN

E

STIFFNESS

0.1026 N/m

FViscosity

A mixture containing identical quantitiesof polystyrene, ethyl-hexyl-acrylate andstyrene-butadiene-rubber was spin coatedonto glass. The topography of the sample
acquired in AC mode reveals a threelevel structure (A). The simultaneously
recorded phase image shows the finestructure of the mixture (B).
DPFM curves showing the force signal
vs. time (D) were recorded from threeareas as marked with the correspondingcolored crosses in the phase image. Forimaging, DPFM curves were measured
at each pixel.
Differences in the DPFM curves generatecontrasts in the images, showing
adhesion (C), stiffness (E), viscosity (F),tip penetration depth (G) and adhesionenergy (H).

0 N/m

G

PENETRATION DEPTH

30 nm

HAdhesion Energy

J

RAMAN SPECTRA

10 nm

I

RAMAN

Concurrent with the AFM measurements
the Raman spectra were recorded at eachpixel using the alpha300 RA, a combined
Raman-AFM instrument.
Spectral analysis reveals the threecomponents of the blend, here displayedin red = EHA, green = PS and blue =mixed spectrum of EHA/SBR (I). From
all data a false color-coded Raman image
was generated (J). Correlating AFM andRaman images shows that theuppermost and stiffest features of thesample appear to be PS while thethinnest areas contain EHA only.

JWITec AFM | 09

AFM Study of Collagen Fibers
Type I collagen is present in many animal tissues
such as bone. It is an elongated, highly organizedstructure consisting of many long molecules
coiled around each other displaying a typicalbanding pattern.

ABAC Mode(A) AC Mode AFM phase image of a collagen fiber.
(B) Height profile of the fiber along the blackline in (A). The profile reveals regular bands 63 nm
in width.

Simultaneous Raman-AFM Measurement
AVideo Imagecantileverlaser spot
lithographed contour

B

17 nm

AFM

0 nm

Nanolithography in GaAs simultaneouslyimaged by Raman-AFM.

CAFM

17 nm

0 nm

(A) Video image with simultaneous cantilever, laser
spot and sample view. (B) AFM topography image.
(C) 3D representation of AFM topography zoom-inimage. (D) Raman intensity image. Image
parameters: 75 x 75 μm2; 200 x 200 pixels;
0.07 s integration time per spectrum.
(E) 3D Raman-AFM image overlay.

DRaman10 CCD cts

0 CCD cts

10ERaman-AFM Overlay

10 CCD cts

0 CCD cts

Combined Raman-AFM Measurement
The modular and flexible design of the WITec alpha300 microscope series
guarantees easy and cost-effective upgrade and extension possibilities. WITec’s
product line incorporates nearly all scanning probe and optical microscopy
techniques to meet individual requirements. Each WITec alpha300 model canbe equipped with new functionalities either as built-in features or as laterupgrades. The WITec hardware and software environment is used for all
features or upgrades, ensuring the best possible compatibility and ease of use.

AB

20 nm

The objective turret is rotated to change
from AFM to Raman imaging mode.

C0 nm

Correlative Raman-AFM image of exfoliated graphene on a silicon substrate. (A) Color-coded Raman
image showing monolayer (red), bi-layer (blue) and multi-layer (green) graphene.
(B) AFM topography image (AC mode). (C) Height profile of a cross section along the black line
indicated in (B). The height variation between one and two layers is approx. 0.3 nm.

A

10 nm

B

0 nm

Correlative Raman-AFM study of a CVD-grown graphene layer. (A) AFM topography image,5 x 5 μm2. (B) Raman image of the same area taken and overlaid with the AFM image.
The different colors indicate layers and wrinkles in the graphene film.

A

10 nm

0 nm

C compressivetensile

B
Correlative Raman-AFM image of wood extractives.
Color-coded Raman image of a cellulose fiber depicting
cellulose (green, blue) and hexane extract (red).
Inset: High-resolution AFM phase image.

Correlative Raman-AFM measurement investigating stress in silicon via Vickers
indent. (A) 10 x 10 µm2 AFM topography image around a Vickers indent.
(B) AFM depth profile view. (C) Corresponding Raman image revealing
the areas of stress in silicon.

WITec AFM | 11We take care
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